How to Make It
Heat grill to high; oil grill. Season the fish with the coriander, ½ teaspoon salt, and ¼ teaspoon pepper and grill until cooked through, 1 to 2 minutes per side. Break into pieces.
In a medium bowl, toss the radishes and cucumber with the lime juice, oil, and ¼ teaspoon each salt and pepper. Serve the fish in the tortillas with the cucumber relish, cilantro, sour cream, and lime wedges.